1. Field of the Invention
The invention relates to the launch and achievement of a desired trajectory above the earth by a spacecraft and, more particularly, to a method for reducing the upper stage mass by an amount sufficient to make stage recovery for reuse feasible and cost efficient.
2. Description of the Related Art
A fundamental objective in designing and building spacecraft using engines powered by a fluid bipropellant comprising constituents such as an oxidizer and a fuel and launching the spacecraft from the earth is to make optimum use of the proportion of total mass carried aloft from the earth with the spacecraft which comprises bipropellant constituents for maneuvering the spacecraft once it has reached the relatively low gravity environment above the earth.
With the advent of the space shuttle, there has also been an impetus to construct a spacecraft with a reusable upper rocket stage which is recoverable by the space shuttle. In order to make such a reusable rocket stage truly feasible, its overall mass must be kept at a minimum by insuring complete and efficient use of the bipropellant.
For a typical geosyncharonous orbiting spacecraft powered by an engine using a fluid bipropellant, for example, the bipropellant may comprise approximately 75% of the combined weight of the spacecraft and the bipropellant. A fluid bipropellant powered spacecraft launched from the space shuttle for geosynchronous orbit about the earth ordinarily requires at least enough bipropellant to propel the spacecraft from a relatively low parking orbit about the earth to a generally elliptical transfer orbit, to propel the spacecraft from a transfer orbit to a substantially circular geosynchronous orbit and to perform station-keeping maneuvers during the operational lifetime of the spacecraft.
In the past, however, several factors have militated against the reduction of overall spacecraft mass and the efficient usage of spacecraft bipropellant. For example, enough spacecraft structure has been necessary to support the spacecraft bipropellant during the trip from the surface of the earth to the relatively low gravity environment above the earth during which high accelerational forces are experienced by the spacecraft and its bipropellant. Furthermore, enough surplus bipropellant often has been carried aloft to compensate for inaccuracies in the calculated utilization of the spacecraft bipropellant.
In earlier spacecraft launches, fluid bipropellant usually was carried aloft within tanks supported by support structure integral to the spacecraft. During launch from earth to the relatively low gravity environment above the earth, the rapid acceleration and vibration of the fluid bipropellant often resulted in loading of the bipropellant with forces equal to many times the force that the earth's gravity would exert on the bipropellant if it were at rest on the surface of the earth. Consequently, tanks containing the bipropellant and support structure supporting it had to be sturdy enough to withstand such high loading. Unfortunately, sturdier tanks and support structure generally were more massive. Thus, the tanks and support structure of earlier spacecraft had to be massive and sturdy enough to withstand the high loading of the bipropellant during the launch.
In the past, a spacecraft often was staged to reduce its overall mass after it entered the relatively low gravity environment above the earth. For example, spacecraft were built which, during the transfer orbit, staged the spacecraft motor which propelled the spacecraft from the parking orbit to the transfer orbit.
Furthermore, in the past, various techniques have been employed in order to more efficiently utilize the bipropellant in order to avoid surpluses. For example, during the firing of a rocket engine, the rate of consumption of each bipropellant constituent has been measured, and its flow rate to the rocket engine has been adjusted accordingly in order to achieve more complete consumption of both bipropellant constituents. Furthermore, in the case of some spacecraft of the type which have had large numbers of launchings, sufficient data on their rocket engine in-flight performance has been compiled to provide a relatively accurate estimate of how much of each bipropellant constituent is needed for a given mission.
While earlier techniques for efficiently utilizing fluid bipropellant generally have been successful, there have been shortcomings with their use. For example, the measurement and adjustment of a bipropellant constituent's flow rate during the firing of a rocket engine often cannot be performed with sufficient accuracy. Furthermore, when a type of spacecraft has not had the benefit of numerous launchings in which to compile bipropellant consumption rate statistics, there may be insufficient data to accurately predict the rates of consumption of the bipropellant constituents during a particular mission.
Thus, there has been a need for a method for launching a spacecraft with a recoverable rocket stage from the earth and for achieving a desired trajectory above the earth while reducing the overall mass of the spacecraft dedicated to supporting the bipropellant during the launch and while efficiently utilizing the bipropellant while the spacecraft is transported from a parking orbit to a geosynchronous orbit. The present invention meets this need.